The present invention relates to a control device of an electric power steering system for a motor vehicle.
There is known an electric power steering system for a motor vehicle wherein a steering force is assisted by the output of an electric motor for reducing the steering force. However, in the steering system of this kind, a response delay of an electric signal transmitted through an electric circuit produces a time delay between the detection of the steering force by a torque sensor and the generation of the assisting force in the electric motor. The time delay causes resistance force against outer disturbance to reduce and thus self-oscillations may occur when the steering wheel is turned while the motor vehicle stops. Furthermore, under the effect of inertia of the electric motor and friction of the speed-reduction mechanism, the response characteristics of the power steering system is reduced, and then the driver feels uncomfortable.
In order to overcome the above described difficulty, a device wherein a signal proportional to the differential of the torque signal is added to the torque signal of the steering shaft has been proposed by the applicant of the present invention, and is now published as Japanese Pat. Laid-Open No. 115771/1986. However, if the control of the steering wheel is too sensitive and the assist force of the electric motor is large even at a small steering angle, the steering force would be reduced and light, and the response characteristic of the steering operation would be much improved, while the motor vehicle is running on a straight road. To the contrary, if the steering wheel is turned greatly and quickly, the steering operation becomes too sensitive, and adverse effect of over-steering tends to occur.